The present invention relates to the field of antenna coupling circuits.
Manufacturers of television receivers and the like are becoming increasingly concerned with safety. One area of considerable importance concerns apparatus for isolating the antenna of a receiver from the receiver itself so that excessively high energy discharges due to chassis potential and lightning are inhibited from being coupled to the receiver from the antenna.
While transformers are known (see, for example, U.S. Pat. No. 3,449,704) which may be used to DC isolate a receiver from an antenna network, these transformers have limited amplitude versus frequency response characteristics and are therefore inadequate for coupling RF signals in both the VHF and UHF ranges from the antenna to RF signal processing circuits of the receiver. As a result, separate transformer devices for each of the VHF and UHF ranges may be required.
Also known is an antenna coupling network utilizing transmission lines (see U.S. Pat. No. 2,757,343) which are constructed without a direct connection between input and output to purposely provide inefficient coupling between an antenna network and an RF circuit. In such an arrangement, the RF circuit must be tuned at each channel in order to provide adequate RF signal coupling. Therefore, such arrangements cannot readily be used between an antenna network and a VHF/UHF signal splitter which is fixed tuned.
Other techniques for coupling antennas utilizing both transmission line and transformer techniques to produce broadband transmission line transformers are know (see, for example, an article entitled, "Some Broad-Band Transformers", by C. L. Ruthroff, appearing in Proceedings of the IRE, Volume 47, August, 1959, pages 1337-1342 and U.S. Pat. No. 2,865,006). Unfortunately, these arrangements all include a direct connection between input and output terminals and are therefore not useful as antenna isolation devices.